Compact depressed electron beam collector

ABSTRACT

A rugged and compact electron beam collector structure for velocity modulation or other high power electron beam vacuum tubes is characterized by means for providing high electrical voltage insulation along a low thermal impedance heat flow path. The device employs a central beam collector as a core flexibly supported within a hollow cylindrical insulator in turn flexibly supported within a vacuum envelope.

O Unlted States Patent 11 1 [111 3,717,787

Doyle 1 1 Feb. 20, 1973 [541 COMPACT DEPRESSED ELECTRON 3,348,08810/1967 Allen, Jr ..313 30 BEAM COLLECTOR 3,471,739 10/1969 Espinosa3,274,429 9/1966 Swiadek ..3l3/46 [75] Hawthorne 2,955,225 10/1960Sterzer ..315/5.38 x [73] Assignee: Sperry Rand Corporation, New

York, NY. Primary Examiner-Herman Karl Saalbach AssistantExaminer-Saxfield Chatmon, Jr. [22] F1led. Aug. 19, 1971 Attorney s C.Yeaton [21] Appl. No.: 173,053

[57] ABSTRACT U-S. Cl. 3 A rugged and compact electron beam collectortruc- 5 ture for velocity modulation or other high power elec- [51] It.Cl .1101] 25/34 tron beam vacuum tubes is characterized by means for[58] held of Search "315/3152 5-38; 1 providing high electrical voltageinsulation along a low 313/46 thermal impedance heat flow path. Thedevice employs a central beam collector as a core flexibly sup- [56]References and ported within a hollow cylindrical insulator in turnUNITED STATES PATENTS flexibly supported within a vacuum envelope.

3,626,230 12/1971 Stewart ..313/46 9 Claims, 4 Drawing Figures PATENTEDFEB 2 01975 ShEEI 1 [3F COMPACT DEPRESSED ELECTRON BEAM COLLECTORBACKGROUND OF THE INVENTION 1. Field of the Invention The inventionpertains to means for improving the efficiency and life of operation ofcompact electron beam power tubes and more particularly concerns arugged, thermal-shock-proof electron beam collector of compact designfor operation at depressed potentials in a velocity modulation highfrequency power vacuum tube.

2. Description of the Prior Art Several types of high frequency powervacuum tubes employ electron beams having high density electron currentsdriven at high velocities, such as velocity modulation tubes of thetraveling wave and klystron types. In these devices, production andacceleration of the electron beam occurs in a cathode-anode region. Theelectron beam then passes into a separate region in which its kineticenergy is used to amplify high frequency electromagnetic oscillations.In early designs of such tubes, the electron beam, still having quitehigh kinetic energy, passed on out of the high frequency interactionstructure to be dissipated in a third region as heat in a collectorelectrode held at the same potential with respect to ground as theinteraction structure. The power wasted as heat directly caused lowefficiency of operation and made necessary the use of additional powerfor operating fluid cooling means to hold the temperature of thecollector at a reasonable operation temperature level. The high velocityelectrons striking such a collector interior often also generatedintensive x radiation, making heavy and expensive shielding a healthprotection necessity.

The over-all efficiency of such beam tubes may be considerably increasedby the use of specially designed electron beam collectors operated atpotentials considerably below the potential of the high frequencyinteraction structure. Such collectors are known as depressed collectorsand permit improved use of the total kinetic energy of the electronbeam. Also, with greatly reduced heating of the collector, considerablyless power is lost for cooling the collector and simple air coolingsystems may be used in place of complex liquid cooling systems.X-radiation is also reduced, permitting reduction in shielding againstits destructive properties.

Depressed collectors have a number of generally conflicting anddifficult requirements which have, in the past, made their constructionin compact and inexpensive form hard to achieve. The large electricalpotential difference between the high frequency interaction structure ofthe tube and the depressed collector requires provision of adequate highvoltage insulation between the two elements. The structure must at thesame time feature a low thermal impedance heat path from the core of thecollector to an external heat sink. The metal and insulator parts of thecollector system are subjected to high temperature-gradients and to hightemperature-gradient rates of change or thermal shock. Repeated cyclesfrom the zero to the operating thermal gradient state must be toleratedif the vacuum tube is to exhibit satisfactory operating characteristicsover an acceptable life. Prior art designs for vacuum tubes withdepressed collectors are notoriously prone to abrupt failures, duringmanufacture as well as in the field, because of the above-mentionedproblems. Rupture of bonds between insulator and metal parts is a commonoccurrence because of thermal shock in operation or because of simplemechanical shock during handling of the tube.

SUMMARY OF THE INVENTION The present invention is a rugged and compactelectron beam collector electrode system of the depressed potential typefor application in velocity modulation vacuum tubes, such as travelingwave tubes and klystrons, and in other beam power tubes for improvementin their operational life and efficiency. The invention is characterizedby the presence of a collector electrode system operated at potentialsbelow the potential of the electron tube high frequency interactionstructure so that a significant portion of the kinetic energy remainingin the electron beam as it exits from the high frequency interactionregion may be recovered, rather than lost as heat. The novel depressedelectron beam collector electrode system is constructed of a flexiblecylindrical element and an array of flexible metal elements bonded to ahollow cylindrical ceramic element in a manner providing reduced shearforces on ceramic-to-metal bonds and accordingly affording long-lifeoperation of the vacuum tube assembly.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section view of apreferred embodiment of the invention.

FIGS. 2 and 3 are respective plan and end views of a portion of thedevice of FIG. 1.

FIG. 4 is an end view of the structure of FIGS. 2 and 3 prepared forinsertion in the structure of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the noveldepressed potential electron beam collector in use in a representativehelix traveling wave tube of the general type illustrated in the J. L.Rawls patent application Ser. No. 54,943 for a Depressed Electron BeamCollector, filed July 15, 1970, issued May 9, 1972 as patent 3,662,212,and assigned to the Sperry Rand Corporation. While FIG. 1 thereforerepresents a particular application of the invention, it will berecognized by those skilled in the art that other known high frequency,slow wave propagation elements may be substituted for the helix highfrequency interaction circuit shown in FIG. 1. Further, it is to berecognized that other types of high frequency interaction circuits maybe substituted for the helix, including the cavity resonator systemscharacteristic of the klystron, such alternative arrangements beingillustrated, for example, in US. Pat. No. 3,172,004, entitled DepressedCollector Operation of Electron Beam Device, issued Mar. 2, 1965 to R.J. Von Gutfeld and C. C. Wang and assigned to the Sperry RandCorporation.

In the apparatus of FIG. 1, it is understood that a conventionalelectron emitting cathode is held at a negative potential with respectto ground by a suitable cathode supply source. Consequently, a linealelectron beam of circularly symmetric character is projected through anaperture in a grounded anode and then flows in conventional energyexchanging relation through a grounded high frequency energy exchangingcircuit device or helix 7 having respective input and output terminalsof which only output coaxial line terminal 6 is shown. Amplified highfrequency energy is' coupled fromhelix 7 at junction 4 to the innerconductor 9 of output coaxial terminal 6. Electrons passing out of helix7, after exchanging energy with the traveling high frequency fieldswithin helix 7, are collected by electron beam-collector electrode 10.Such is accomplished with the collecting electrode 22 of collector 10 ata potential considerably depressed with respect to the ground potentialof helix 7 by virtue of a conventional collector potential source (notshown) one side of which may be connected via lead 14 to collector 10 inthe conventional manner.

As an example of operating potentials applied for operation of atraveling wave tube embodying the novel electron beam collector, thecathode may be operated at about -9,000 volts, the anode and helix 7 atground potential, and the collector electrode 22 at about -4,50O volts.The cited electrical potential values are to be taken merely asrepresentative examples and are not necessarily intended to describelimiting or optimum values for operation of a traveling wave, klystron,or other beam power tube according to the invention.

FIG. 1 represents a preferred embodiment of the invention in which theapertured diaphragm may be spoken of as lying in a plane between old andnew parts of a traveling wave amplifier tube embodying the invention. Asis generally illustrated in the above mentioned Von Gutfeld et al Pat.No. 3,172,004, diaphragm 20 and parts to the left of diaphragm 20 suchas vacuum envelope 19 are conventional parts to be recognized as beingassembled in the manner that they are customarily assembled in prior arttraveling wave amplifiers. Parts to the right of diaphragm 20 make upthe novel depressed collector structure of the present invention. It isto be observed that aperture 21 of diaphragm 20 is aligned with the axisof the electron beam and that the electron beam, after having interactedwith high frequency fields such as those within helix 7 passes throughaperture 21 of FIG. 1 into collector 10.

The electron beam is actually stopped or collected by the generallysymmetric interior walls of hollow electron beam-collector core 22,these inner walls being provided by cooperating axially aligned andaxially extending interior sections forming wall portions 220 and 22b.The core 22 may be made of a material such as annealed oxygen-freecopper. Wall portion 22a is in the form of a frustrum of a cone. It isjoined at its small diameter end to a circularly cylindric wall portion22b. Wall portion 22b is then joined to a conical end wall22c.

The collected electron beam current may be drawn from hollow core 22 vialead wire 14 which may also be comprised of copper and may be fastenedby any of several known suitable means at junction 14a to the outer endof core 22. The collected current, as suggested previously, may be drawnoff at a voltage which is negative with respect to the voltage on helix7 and which is typically 40 to 60 per cent of the voltage supplied tothe beam forming cathode.

The progressively decreasing size of axially extending interior walls22a and 22b permits each wall section to collect substantially the samefraction of the total electron beam current. Thus, the portions of thetotal remaining kinetic energy of the electron beam converted to heat atwall sections 22a, 22b, and 220 may be substantially equal. As aconsequence, such heat is relatively evenly distributed along the lengthof core 22.

Core 22 is supported within an outer jacket or generally cylindricalvacuum envelope 24, which may be supplied with a base 5 adapted for useas a heat sink, and which may also be constructed of a metal such asoxygen-free copper. One end of wall 24 is closed by the diaphragm 20sealed at its periphery to wall 24, for example, by a circular brazedjunction 26.

A drawn cup-shaped metal end wall or cap 25 is fastened at the oppositeend of vacuum envelope 24 by a circular brazed vacuum tight junction 27.End wall or cap 25 is of sufficient volume to accommodate collector lead14, which is formed with a right angle bead and projects out of theinterior of cap 25 through insulator 29. Insulator 29 may be formed ofany of several available ceramic materials having good high voltageinsulation properties and adapted to form a vacuum tight seal with themetal of cap 25 at circular junction 30. Lead wire 14 is similarlysealed within a hole in insulator 29 at surface 28. Although othermaterials may be used, certain nickel-iron-chromium alloys have beenfound to be useful for cap 25, since they are capable of being generatedby pressure drawing and since they readily form vacuum tight seals withcertain ceramic materials.

From the foregoing, it is apparent that outer wall 24, cap 25, andinsulator 29 complete the vacuum envelope means of the high frequencytube structure embodying the invention. Wall 24 has additionalsignificant functions, in that it cooperates in the support of core 22by means providing high electrical voltage insulation along radial, lowthermal impedance, heat flow paths from core 22 to wall 24, as will bedescribed. For dissipating such heat, the vacuum wall 24 may aspreviously noted be supplied with an enlarged flat or otherwise shapedportion 5 capable of being mounted on a suitable heat sink. It is clearthat alternative external heat sink arrangements may readily be appliedby those skilled in the art. It will also be seen that vacuum jacket orwall 24 is arranged in an advantageous manner so that substantially allparts of the collector system are within its interior and therefore arenot accessible, reducing the possibility of injury to persons byelectrical shock and of mechanical damage to the collector parts.

Heat from collector core 22 is conducted directly in a plurality ofradial paths from the outer cylindrical surface 34 of core 22 by aregular series of annular, flexible, and generally radial supportelements 35a, 35b, 35c, 35d, 3514. Each such oxygen-free copper supportelement comprises, as in the instance of support element 35a, an annularreentrant flexible portion 36, a first or outer annular ring shapedportion 37, and a second or inner ring shaped portion 38 substantiallyparallel to outer ring 37. Each such successive flexible portion 36 issupplied with holes, of which only holes 330, 33f, 33l, and 33p are seenin FIG. 1. Other similar annular support elements, such as element 35b,are equipped with similar holes which may lie in planes other than theplane of the drawing and are therefore not seen in FIG. 1. The array ofholes including holes 330, 33f, 331, 33p, and others not seen in FIG. 1permits passage of gas from the interior of the collector structure 10,such as gas from the interior of end cap 25,

when the electron tube interior is being evacuated.

Each annular support element, such as annular support element 35a, hasrespective parallel outer surfaces associated with the respective ringportions 37 and 38 for the purpose of permitting bonded seals to be madethereto. For example, the outer surface of outer ring shaped portion 3'!is sealed to the inner cylindrical surface 41 of vacuu'm envelope 24.Similarly, the outer surface of each successive ring shaped portion 38of all of the flexible support elements, including, for example, supportelement 35a, is sealed to the outer cylindrical surface 42 of insulator22. Thus, an array of annular flexible support elements 35a to 35u isformed,

each element being solidly afflxed to cylindrical wall 41, and eachelement affording an outward and radial heat flow path. The severalseals between flexible supports 35a to 35a and wall 41 may be made byconventional brazing methods using a gold-copper solder alloy.

The outer surface of the inner ring shaped portion 38 of annular support35a is sealed to the cylindrical outer surface 42 of a relativelythick-walled hollow tube 43 of electrically insulating, thermallyconducting nature, preferably fabricated of a material such as berylliumoxide. Similarly, the outer surface of each successive outer ring shapedportion 37 of all of the flexible support elements, including forexample support element 35a, is sealed to the inner surface 41 of theouter vacuum jacket 24. Thus, an array of flexible support elements 35ato 3514 is solidly afflxed to the electrically insulating cylindricalwall 42 in radially outward heat transfer relation. The several sealsmade to the flexible supports 35a to 35u may be made by conventionalmethods, such as by the use of a gold-coppersolder or brazing alloy.

The electrically insulating and radially heat conducting structure ofthe novel electron beam collector system is further completed by a thinand generally symmetric tubular sleeve structure 44 resiliently afflxedbetween the outer surface 34 of the collector core 22 and the innercylindrical surface 50 of the thermally conducting, electrical insulatortube 43. As seen in FIGS. 2 and 3, the resilient sleeve 44 is fabricatedfrom a thin oxygen-free copper sheet 45. It may be made in any ofseveral conventional ways, such as illustrated in FIGS. 2 to 4, whereinFIG. 2 represents a flat sheet 45 of oxygen-free copper material inwhich an array of dimples has been pressed in a conventional manner by astandard material press or roller operation. Each dimple, such as dimple46 in FIGS. 2 to 4, is one dimple of a substantially regular array ofdimples, which array may take any of several convenient geometric forms.In any event, the dimpling tool is preferably operated from one side ofthe suitably supported sheet 45 so that the material in the vicinity ofeach contacting punch face is moved out of the normal plane of sheet 45to form the raised or extruded dimples 46, for example. After dimples 46are formed in the flat sheet 45, sheet 45 may be rolled into the shapeof a cylindrical tube about a suitable mandrel to form the hollow tube44 shown in FIG. 4. The tube 44 may be left with an unclosed gap 47, ifdesired, so as to permit its ready insertion into insulating tube 43.

V In construction of the novel electron beam collector, tube 44 isinserted between the collector core 22 and the beryllium oxide tube 43is afflxed in place by a conventional furnace brazing process using, forinstance, a

conventional gold-copper brazing or solder alloy. In

considering the dimensions selected for the illustration of sleeve 44 inFIGS. 1 to 4, it will be appreciated that the proportions shown areshown merely for convenience in making the several drawings clear; forexample, only FIGS. 2 and 3 are substantially to the same scale and noneof the figures necessarily presents dimensions which would be used inactual practice. It will be understood that the brazing of tube 44between collector core 22 and the beryllium oxide tube 43 may beaccomplished, as is often the practice, at the same time that brazedjoints between other similarly constructed seals are made. As is seenfrom FIGS. 1 and 2, the dimples 46 may be arranged in offset arrayfashion such that gases in the volume between tubular insulator shell 43and the collector core 22 are readily exhausted during evacuation of thecollector prior to scaling off the tube.

The support of collector core 22 within insulator tube 43 is furthercompleted adjacent electron beam aperture 21 by a shaped collector coreelectron beam entrance element 40 having an electron entrance aperture39 shaped and spaced relative to aperture 21 so as to permit entrance ofthe expanding electron beam into the interior of core 22. For thispurpose, it is seen that the diameter of aperture 39 may becorrespondingly greater than the diameter of aperture 21. Element 40 maybe constructed of oxygen-free copper and has an annular expanded region48 with a ring surface 49 adapted to be brazed or soldered to the innersurface of insulator tube 43. Y

The composite depressed collector device is seen to have severalsignificant characteristics. Its compact construction provides fullyadequate electrical insulation of parts at high electrical potentialfrom ground potential and at the same time features a low thermalimpedance path for heat flow to an external thermal sink. Problems withexcessive relative differential thermal expansion of ceramic and metalparts are significantly reduced.

A primary beneficial feature of the invention lies in the presence offlexibility or relative freedom of movement of the parts of the compactassembly. For example, when the electron collector is cold, allcollector parts are at substantially the same temperature. When beampower is turned on, the electron collector core 22 heats more rapidlythan its surroundings and increases in both radial and axial dimensions.The outwardly extending arrays of flexible support elements 35a, 35b,35a are made of a soft flexible material, easily flexed, thus allowingmany repeated cycles of such-dimensional changes without placingdestructive shearing stresses on the ceramic-to-metal bonds. In asimilar manner, the dimpled shell 44 adjacent the electron beamcollector core 22 is readily flexed as temperatures and temperaturegradients rapidly change, again allowing many repeated heating andcooling cycles of the beam collector system without fear of damage toceramic-to-metal seals.

The compact collector configuration is such as to permit maximum axialtranslation of the collector core 22 with respect to the envelope 24,for instance, and also substantially to convert the tendency towardradial expansion'of the assembly into axial expansion, thus furtherreducing shear stresses on ceramic-to-metal bonds. Among otheradvantageous features of the compact collector structure, such asrelative freedom from exposure of personnel to high voltage parts, isthe feature that all surfaces of the insulator tube 22 are protectedfrom exposure to moisture and dust. Similar features of the interiorportions of the collector protect interior surfaces of the insulator 22from degradation by deposition thereon of material evaporated orotherwise removed from the inner walls of core 22 by highly energeticelectrons. I

It is seen that the invention represents a significant improvement overthe prior art, permitting realization of a rugged, compact, shock proofelectron beam collector such as may be used for operation at depressedpotentials. The rugged structure incorporates novel features assuringlong life of an electron beam power tube under repeated cycles ofoperation without damage to cooperating metal and insulator parts of thecollector and to bonds between those parts. Expansion effects arebeneficially directed so that shear stresses on such metal-to-insulatorbonds are significantly reduced, permitting long life of the vacuum tubestructure even under severe operating conditions.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than of limitation and that changes within thepurview of the appended claims may be made without departure from thetrue scope and spirit of the invention in its broader aspects.

1 claim:

1. A linear electron beam device comprising:

electron beam forming means,

high frequency circuit means in energy exchanging relation with saidelectron beam,

hollow electron beam collector means spaced from said high frequencycircuit means and having an axially extending interior surface means forcollecting said electrons and an axially extending outer surface means,

vacuum envelope means supporting said electron beam forming means andsaid high frequency circuit means,

flexible support means for supporting said electron beam collector meansin electrically insulated,

thermally conducting relation within said vacuum envelope meanscomprising:

thin walled hollow metal tube means sealed to said axially extendingouter surface means and having an array of radially extruded dimplesraised above said axially extending outer surface means, thick walledelectrical insulating, thermally conducting tube means having axiallyextending inner and outer concentric surface means, said thick walledtube means axially extending inner surface means being sealed to saiddimples, and an array of annular flexible metal support means sealed tosaidaxially extending outer surface means of said thick walled tubemeans, said array being additionally sealed within said vacuum envelopemeans in heat exchanging relation therewith.

2. Apparatus as described in claim 1 wherein said electron beamcollector means is coupled to electrical conductor means passing ininsulated relation through said vacuum envelope means and adapted to beoperated at a potential negative with respect to said high frequencycircuit means and positive with respect to said electron beam formingmeans.

3. Apparatus as described in claim 1 wherein said annular flexiblesupport means each comprise:

outer ring-shaped wall means having a first surface adapted for scalingto said vacuum envelope means,

inner-ring-shaped wall means substantially parallel to said outerring-shaped wall means and having a second surface adapted for scalingto said thick walled tube means, and

annular reentrant flexible means forming a continuous link between saidouter and inner ring-shaped wall means.

4. Apparatus as described in claim 3 wherein said annular reentrantflexible means are provided with apertures for permitting gas flowtherethrough during manufacture of said electron beam device.

5. Apparatus as described in claim 4 wherein said collector means, saidthin walled metal tube means, and said array are made of oxygen-freecopper.

6. Apparatus as described in claim 5 wherein said thick walled tubemeans is made of beryllium oxide.

7. Apparatus as described in claim 4 wherein said dimples of said thinwalled hollow metal tube means are arranged in a substantially regulararray in mutually spaced relation for permitting gas flow therethroughduring manufacture of said electron beam device.

8. Apparatus as described in claim 1 wherein said hollow electron beamcollector means includes electron beam collector entrance aperturedefining means sealed in abutting relation with said thin walled hollowmetal tube means to said axially extending inner surface means of saidthick walled tube means.

9. Apparatus as described in claim 8 wherein said entrance aperturedefining means is made of oxygen-free copper.

1. A linear electron beam device comprising: electron beam formingmeans, high frequency circuit means in energy exchanging relation withsaid electron beam, hollow electron beam collector means spaced fromsaid high frequency circuit means and having an axially extendinginterior surface means for collecting said electrons and an axiallyextending outer surface means, vacuum envelope means supporting saidelectron beam forming means and said high frequency circuit means,flexible support means for supporting said electron beam collector meansin electrically insulated, thermally conducting relation within saidvacuum envelope means comprising: thin walled hollow metal tube meanssealed to said axially extending outer surface Means and having an arrayof radially extruded dimples raised above said axially extending outersurface means, thick walled electrical insulating, thermally conductingtube means having axially extending inner and outer concentric surfacemeans, said thick walled tube means axially extending inner surfacemeans being sealed to said dimples, and an array of annular flexiblemetal support means sealed to said axially extending outer surface meansof said thick walled tube means, said array being additionally sealedwithin said vacuum envelope means in heat exchanging relationtherewith.
 1. A linear electron beam device comprising: electron beamforming means, high frequency circuit means in energy exchangingrelation with said electron beam, hollow electron beam collector meansspaced from said high frequency circuit means and having an axiallyextending interior surface means for collecting said electrons and anaxially extending outer surface means, vacuum envelope means supportingsaid electron beam forming means and said high frequency circuit means,flexible support means for supporting said electron beam collector meansin electrically insulated, thermally conducting relation within saidvacuum envelope means comprising: thin walled hollow metal tube meanssealed to said axially extending outer surface Means and having an arrayof radially extruded dimples raised above said axially extending outersurface means, thick walled electrical insulating, thermally conductingtube means having axially extending inner and outer concentric surfacemeans, said thick walled tube means axially extending inner surfacemeans being sealed to said dimples, and an array of annular flexiblemetal support means sealed to said axially extending outer surface meansof said thick walled tube means, said array being additionally sealedwithin said vacuum envelope means in heat exchanging relation therewith.2. Apparatus as described in claim 1 wherein said electron beamcollector means is coupled to electrical conductor means passing ininsulated relation through said vacuum envelope means and adapted to beoperated at a potential negative with respect to said high frequencycircuit means and positive with respect to said electron beam formingmeans.
 3. Apparatus as described in claim 1 wherein said annularflexible support means each comprise: outer ring-shaped wall meanshaving a first surface adapted for sealing to said vacuum envelopemeans, inner-ring-shaped wall means substantially parallel to said outerring-shaped wall means and having a second surface adapted for sealingto said thick walled tube means, and annular reentrant flexible meansforming a continuous link between said outer and inner ring-shaped wallmeans.
 4. Apparatus as described in claim 3 wherein said annularreentrant flexible means are provided with apertures for permitting gasflow therethrough during manufacture of said electron beam device. 5.Apparatus as described in claim 4 wherein said collector means, saidthin walled metal tube means, and said array are made of oxygen-freecopper.
 6. Apparatus as described in claim 5 wherein said thick walledtube means is made of beryllium oxide.
 7. Apparatus as described inclaim 4 wherein said dimples of said thin walled hollow metal tube meansare arranged in a substantially regular array in mutually spacedrelation for permitting gas flow therethrough during manufacture of saidelectron beam device.
 8. Apparatus as described in claim 1 wherein saidhollow electron beam collector means includes electron beam collectorentrance aperture defining means sealed in abutting relation with saidthin walled hollow metal tube means to said axially extending innersurface means of said thick walled tube means.